Mechanical equipments and storage vessels (tanks) operated with a gaseous medium, in particular with pressurized air, are regulated by safety valves (blow-off valves) by releasing air in case of overpressure in a proper quantity from the delivery duct of the equipment or the tank into the atmosphere for a short span of time. This process is known as blowing off. The air streaming or accelerating through the valve generates noise under the influence of non-stationary forces when coming into contact with solid bodies. This noise lasts as long as the pressure in the tank or duct is being reduced to the required value and the blow-off valve is open. The noise is the result of the conversion of pressure energy in a very short time, and accordingly it is characterized by a high level and a wide band. First of all, turbulence is causing the noise and begins to be effective at about 500 Hz and increases rapidly with increased frequency. At the same time a low frequency noise spectrum also appears which contains the acoustic state of the environment of blow-off too. The peak is mostly formed at the frequency of the fundamental resonance of the closed space--where blow-off is taking place--which decreases with increased frequency. The two fundamental spectra are meeting in the middle-frequency field, where the noise of blow-off is the lowest.
In order to reduce environmental acoustic trauma affecting people, noises of blow-off valves are to be supressed, and therefore, for example, blow-off valves of pneumatic brake systems of vehicles, blow-off valves in pneumatic systems in workshops are to be provided with noise damping devices.
With blow-off valves of known pneumatic systems silencers made of sintered bronze have been used, wherein a part of the noise-energy of the streaming blow-off air is converted into heat by friction. These sound damping devices suppress high-frequency noises, however, they are scarcely effective against low-frequency noises. Their size, and thus their weight and cost depend on the quantity of the air to be blown off during a unit of time. Generally, they are made of cylindrical rods with a considerable space requirement which is to be considered, together with possibilities of installation when planning pneumatic systems.